Method of ceramic brazing



May 26, 1970 J. F. CLARKE 3,513,535

' METHOD OF CERAMIC BRAZING Original Filed Feb. 26. 1964 3 Sheets-Sheet2 May 26, 1970 4 J, CLARKE 3,513,535

METHOD OF cmmmc BRAZING Original Filed Feb. 26, 1964 s Sheets-Sheet aFIG. lo. ii)

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United States Patent U.S. Cl. 29-4731 11 Claims ABSTRACT OF THEDISCLOSURE A method of brazing a ceramic part to a metal part. The metalpart is first bonded, by means of solid-phase bonding techniques, to atrilayer solid-phase bonded brazing sheet and the resultant four-layerstarting material is then formed, such as by punching, cutting or thelike, to the desired shape. The side of the starting material composedof the trilayer brazing sheet is then assembled against the ceramic partand the entire assembly is squeezed together and heated to a temperatureat which the trilayer brazing sheet forms a viscous liquid-phase alloywhich bonds with the ceramic part. The pressure and temperature arethereafter lowered to solidify the alloy in a bond with the ceramicpart.

CROSS-REFERENCE TO RELATED APPLICATION This application is a division ofmy earlier application Ser. No. 347,610, filed Feb. 26, 1964, now US.Pat. No. 3,382,052.

This invention relates to a method of ceramic brazing, and with regardto certain more specific features, to such methods employing an alloycombining a reactive metal with a relatively nonreactive metal, whereinthe reactive metal acts with the ceramic at a brazing temperature topromote good wetting and bonding to the ceramic.

Among the several objects of the invention may be noted the provision ofimproved brazing methods for connecting ceramic such as alumina to metalor to ceramic; the provision of methods of the class described wherein alayered sandwich of thin sheet components for alloy formation isemployed in such a manner as not to complicate required cutting andstamping operations; the provision of brazing methods which willminimize thermally induced stresses and embrittlement at the brazedjoints; and the provision of methods of the class described which willproduce brazed assemblies having better and constant quality andappearance than heretofore obtained. Other objects and features will bein part apparent and in part pointed out hereinafter.

The invention accordingly comprises the elements and combinations ofelements, ingredients and combinations of ingredients, and proportionsthereof, steps and sequence of steps, features of construction,composition and manipulation, and arrangements of parts which will beexemplified in the constructions, products and methods hereinafterdescribed, and the scope of which will be indicated in the followingclaims.

In the accompanying drawings, in which several of various possibleembodiments of the invention are illustrated:

FIG. 1 is a greatly enlarged fragmentary cross section of startingmaterial made according to the invention;

FIG. 2 is an enlarged plan view of one form of intermediate product madeaccording to the invention;

FIG. 3 is an edge view of FIG. 2;

FIG. 4 is a plan view of a ceramic member, to be connected with theproduct shown in FIGS. 2 and 3;

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FIG. 5 is a cross section taken on line 5-5 of FIG. 4;

FIGS. 6 and 7 are cross sections of apparatus for assembling the devicesshown in FIGS. 2 and 4, FIG. 6 illustrating a first assembly step andFIG. 7 a subsequent assembly step;

FIG. 8 is a typical brazed product made according to the invention;

FIG. 9 is a view similar to FIG. 1, illustrating another form of astarting material made according to a modification of the invention; and

FIG. 10 is a diagrammatic cross section illustrating another product inthe process of manufacture according to the invention.

Corresponding reference characters indicate correspond ing partsthroughout the several views of the drawings.

It is known to make a brazing alloy by combining a reactive metal suchas titanium with a relatively nonreactive metal such as nickel. ,Such analloy is useful for connecting a ceramic such as alumina (A1 0 to metalbecause the said reactive metal reacts with the ceramic at the brazingtemperature to promote good wetting and bonding between the brazingalloy and the ceramic member while the brazing alloy adheres to themetal member to which the ceramic is to be connected.

Heretofore the reactive and nonreactive metals according to one methodhave been melted and cast together by conventional means and then formedor cut to a desired shape for making the desired brazed bond. A secondmethod has been to employ in mixed powder form components which werecompacted and sintered and then formed or cut to the desired shape. Athird method has been to solid-phase bond two or more layers of thealloy components which have then been rolled out and cut to the desiredshape. In the latter case the alloy was produced upon heating at thetime that the layers were located in brazing position. The first twomethods are restricted to ductile alloys. The chief disadvantage of allof these prior methods was that the final shaping operation of thebrazing material was required to be performed on very thin material,which was both difficult and expensive. According to the presentinvention, the above disadvantages are obviated.

Hereinafter the term metal will be understood to include metal alloys.It will also be understood that the invention is useful in bondingceramic to ceramic, although the case of bonding ceramic to metal isdescribed by way of example. The term solid-phase bonding means thebonding of two metals without the formation between the metals of anyliquid phase. Typical solid-phase bonding processes are set forth in US.Pats. 2,691,815 and 2,753,623 and their disclosures are incorporatedherein by reference. The term liquid phase is intended to cover eitherthe phase in which the liquid is free-running or viscous, unlessotherwise stated. Since the thicknesses of layers hereinafter describedare very small, they are not to scale in the drawings.

Referring now more particularly to FIG. 1, there is shown at numeral 1 asheet of any suitable base metal which is appropriately to be formed andto which an appropriately formed ceramic part is ultimately to beattached. Preferably, although not necessarily, as will appear, themetal 1 should have a coefiicient of thermal expansion approximatingthat of the ceramic which is to be joined thereto. For example, in thecase of the ceramic alumina (A1 0 a desirable alloy for the base layer 1might be 42% or 46% nickel, with the remainder iron in each case. Or thematerial 1 may be Kovar, which is an alloy consisting of 29% nickel, 17%cobalt and the balance iron, with a trace of manganese. Otherformulations for the layer 1 will suggest themselves in view of thefollowing complete description of the invention.

In accordance with the invention, a brazing sheet is made up by layers3, and 7, consisting for example of silver (Ag), titanium (Ti) andsiliver (Ag) in thicknesses of 0.005 inch, 0.010 inch, and 0.005 inch,respectively. These layers 3, 5 and 7 are preliminarily solidphasebonded to one another according to processes such as described, forexample, in said patents. The 0.020 inch sandwich 3, 5, 7 is thensolid-phase bonded to the desired metal layer 1, which latter initiallymay be 0.075 inch thick. The resulting four-layer, bonded sheet isfinishrolled to 0.015 inch to form a starting material S, as illustratedin FIG. 1. This material is of suificient thickness to be mechanicallyworked to a desired shape, such as by forming, punching, cutting or thelike, without incurring distortions which in the case of very thinmaterials required costly corrective measures. For example it isdifiicult satisfactorily to form, punch or cut thin material such asthose in the trilaminate assembly (3, 5, 7), whether layers 3, 5, 7 areoperated upon individually or as a subassembly. Alternately, the threelayers 3, 5 and 7 and the metal layer 1 may be solid-phase bonded in thesame operation to form the starting material S.

The next step is to form the starting material S so that at least itslayer 1 will have a proper final base shape. Thus, for example, theremay be conveniently and successfully punched from the material S anannular or washer-like form such as shown at 9 in FIGS. 2 and 3. It willbe understood that this form is only one of many that may be employed.The particular one chosen, for example, is to form flanges on aspool-shaped product L as illustrated in FIG. 8. The core 11 of thisspool is formed of the ceramic alumina (FIGS. 4 and 5). The ultimateobject in this example is to braze two of the fourlayer washers 9 toopposite sides of the core 11 to form the spool shape L. In order to dothis the side composed by the layers 3, 5, 7 of each of two washers 9 isaligned and faced in assembly against an end of the core 11. Theassembly is then heated to brazing temperature to form a liquid-phasealloy of layers 3, 5 and 7. Upon cooling and solidification, the bondedproduct such as in FIG. 8 results, wherein the ceramic core 11 has beenprovided with attached flanges composed of the metal 1.

A suitable brazing temperature for material such as above described is2,000 F., for a half hour in a vacuum of 6x mm. Hg. Titanium in theresulting molten silver reacts with the alumina, promoting wetting andbonding. An advantage thus far will be seen to be that the forming andcutting of the brazing components 3, 5, 7 can, in their position ofattachment to the formed material 1, be done along with the forming andcutting of this material. As a consequence, thinner sections of thebrazing alloy can be used than if an attempt were made to out eachindividual layer 1, 3, 5 or 7 to shape prior to any bonding; or if anattempt were made int dividually to form the layer 1 on the one hand anda separate composite layer 3, 5, 7 on the other hand, prior to bondingthe latter (3, 5, 7) to layer 1.

When the layers 3 and 7 have the same melting point as in the case abovegiven, they melt at the same temperature. As a result, the braze metalnot in contact with the ceramic as at 39 in FIG. 8 may ripple enough tocause uneven oxidation upon cooling. This condition would not occur ifthe diameter of 9 were equal to the diameter of core 11. But if in thecase illustrated uneven oxidation is not to be tolerated at 39 in thefinished product, it can be overcome as follows:

The four layers 1, 3, 5, 7 in the completed material S, ready forforming operations, may be composed as follows, for example: layer 1 maybe Kovar or the like, .015 inch thick; layer 3 (Ag), .0004 inch thick;layer 5 (Ti) .0008 inch thick; and layer 7 (a silver-copper alloy; 72%Ag28% Cu), .0008 inch thick. The whole bonded assembly S will th n be.017 inch thick. In such case base 1 is 0.15 inch thick and the attachedtrilaminate assembly (3, 5, 7) is .002 thick. This selection ofmaterials for the layers 3, 5, 7 has the advantage that the layer 3 hasa higher melting point than the layer 7. As a result, the surface of theexposed braze metal as at 39 (FIG. 8) becomes smoother and more evenlyoxidized. Because the layer 3 does not melt at the temperature requiredto melt the layer 7, the layer 3 tends less to form any brittlecompounds with the adjacent layers. Moreover, accurate fillet formationduring brazing is more readily controlled at 13 (FIG. 8).

Sometimes the active metal for layer 5 or 35 (referred to hereinafter),instead of being titanium, can advantageously be otherwise composed.This is for the reason that during the rolling to final size thetitanium sometimes does not deform a constant amount along the length ofthe rolled strip material. This results in localized areas of incorrecttitanium concentration, which may cause the quality of the braze to varythroughout the assembly.

Then when the joint is heated to too high a temperature or for a longertime than necessary to diffuse the titanium to an even concentration,the joint becomes brittle and highly stressed. In such cases there couldbe substituted for the titanium intermediate layer 5 an alloy layer suchas columbium containing 10% titanium and 5% zirconium, which is known asalloy D-36. Suitable designations for the laminate illustrated in FIG. 1would then be as follows: layer 1, Kovar; layer 3, (Ag); layer 5, (alloyD36); layer 7, (72% Ag28% Cu) in a thickness ratio of 18:1:1z2, rolledto an over-all thickness S of 0.017 inch. Upon heating this combinationwith alumina in contact with layer 7 to 1,500" F. for five minutes, abrazed joint is produced which is in some cases superior to oneemploying titanium as the active metal. This is for the reason that theactive alloy D-36 deforms evenly along the length of the strip,resulting in a concentration of active metal and constant quality ofbraze in all areas of the joint. Also, the amount of active metal D-36in the thin layer 5 dissolved in the molten braze metal is limited, toproduce a more ductile joint. Moreover, the thermal expansion of theD-36 alloy better matches the thermal expansion of the alumina than doestitanium. This reduces residual stresses in the joint upon cooling. Infact, minimization of stress can be adjusted at will by controlling theamount of alloy D36 present.

I have found that appropriate control of time, temperatures, and holdingpressure during the heating operation for brazing results in a betterbrazed product. If the brazing temperature is too low, there resultsincomplete production of a liquid phase of the alloy made by layers 3,5, 7 with consequent incomplete wetting of the base metal 1 and theceramic 11. This results in a weak, porous bond. When the temperature istoo high, excessive flow of the completely liquid alloy formed by thecomposite 3, 5, 7 mars the appearance by run-out and limits theusefulness of the metal-ceramic assembly. For example, there may resultan unduly large filleting effect at the regions 13 in FIG. 8 because ofloss of liquid from between members 1 and 11. A solution to this problemis to employ a brazing temperature below that corresponding to theoptimum liquid fio-w temperature of the composite 3, 5, 7. Thus thetemperature is brought only to the point at which the composite 3, 5, 7assumes a viscous liquid phase, as distinguished from a free-flowingliquid phase. In addition, under such a condition, squeezing pressure isapplied. The viscous liquid phase is sufficient to bring about alloying,and the pressure applied during heating forces the viscous alloy topread over the ceramic surface through the desired area without runningout excessively.

Convenient apparatus for obtaining squeezing pressure during heating isillustrated in FIGS. 6 and 7, in which numeral 15 illustrates a jawclamp formed of a material having a low coefiicient of thermal expansionsuch as graphite. A hole 17 is provided in one jaw 19 and a graphitescrew member 21 is threaded through the other jaw 23. A ring of material25, having a high coefficient of thermal expansion, is also provided. At27 is shown a pin adapted to be pushed through the hole 17 to form astacking guide for the ring 25 and the members 9 and 11 (FIG. 6). Thenthe screw-threaded member 21 is screweu down into mere holding contactwith the upper washer 9, the pin 27 being withdrawn (FIG. 7). Theassembly 9, 11, 9 and 25 thus held in the clamp is placed in the heatingfurnace and during a suitable time raised to a temperature correspondingto the viscous liquid phase of 3, 5, 7. This is just below the optimumor liquid flow temperature. This temperature also causes the ring 25 andlayers 9, 11 and 9 to expand and to bring about gradually risingsqueezing pressure as the alloying viscous liquid phase occurs. Thisspreads or pushes the alloy all over the surfaces to be convertedwithout further outflow. Then when the clamp is removed from the furnaceand cooled, solidification of the viscous alloy material occurs, alongwith shrinkage of the ring 25 and layers 9, 11 and 9, whereupon theproduct may be removed and is ready for use as shown in FIG. 8. Itsadvantage then is that of a strong joint without disfiguring run-outthereon of any braze material. Thus in the FIG. 8 form, the fillets at13 are of minimal size. Alternatively, where a sufficient differentialof thermal expansion exists between layers 9, 11 and 9 and jaw clamp 15,the ring 25 may be omitted.

Sometimes it may be desirable to effect bonding between a ceramic memberand a backing material such as 1, wherein said backing material and theceramic member have widely dilferent coeflicients of thermal expansion.In such a case, with certain configurations of final products thethermal stresses produced in the joint of the final product duringcooling may cause bond failures. To overcome this, a buffer metal withan intermediate coefiicient of thermal expansion between those of theceramic member and the backing member 1 can be initially bonded betweensuch member 1 and the bonding trilaminate assembly. This is shown forexample in FIG. 9, wherein five layers are solid-phase bonded to providethe material to be punched, sheared or otherwise formed. At numeral 29is shown part of the backing layer, corresponding to layer 1 in FIG. 1;layer 31 is the buffer layer; and layers 33, 35 and 37 are the componentlayers adapted to form the brazing alloy. It will be understood that allof the layers 29, 31, 33, 35 and .37 are solid-phase bonded to form thematerial from which objects such as 9 are formed for subsequentattachment to ceramic articles such as 11.

Sometimes bonds are required that do not admit of assembling the brazingtrilaminate assembly such as (3, 5, 7) or (33, 35, 37) to the backingmaterial such as 1, prior to mechanical forming operations. Such a caseis illustrated in FIG. 10, wherein it is desired tobraze two metal pins41 (composed, for example, of Kovar) into a hole 43 in a block ofceramic 45. In such case an unbacked trilaminate material (47, 49, 51)is used, having a cross section near that of the pins 41. This isinserted into the space 43 between the pins in the hole 43. In addition,a block of high-expansion metal 53, such as the metal which makes up thering 25 in FIGS. 6 and 7, may be provided if necessary. Then theassembly is placed in a suitable clamp to provide light pressure in thedirection indicated by the arrows 55. This pushes together parts 41, 47,49, 51 in the hole 43. In this condition the assembly is placed in afurnace. Heating is carried out to a point such that a viscous conditionof the alloy from the layers 47, 49 and 51 is produced. As the block 53expands, this viscous liquid-phase material is forced to spread alongall of the available interfaces between members 41 and 45. Thus uponremoval from the furnace a junction is formed between each pin 41 andthe ceramic 45, as well as a junction between the pins themselves.

The form of connection provided by the process illustrated in FIG. doesnot take advantage of any unitary assembly of layers such as 1, 3, 5, 7,inasmuch as the brazing layers 47, 49, 51 are independent of anystiffening base layer. The showing of FIG. 10, does however emphasize anadvantageous feature of the invention, taken in and of itself. This isthe heating under pressure of the alloya'ble trilaminate material onlyto the viscous liquidphase temperature. In this case (FIG. 10), by notcarrying the heating step to a temperature which will cause free liquidflow, there is avoided the condition of excessive run-out and starvationof the alloy in the joint. As will be seen from FIG. 10, excessiveoutflow would weaken the connection between the pins 41 and ceramic 45.In the case of viscous flow under pressure, this does not occur.

Summarizing, the invention has several important features:

1) No punching and handling of extremely thin parts need to be resortedto, such as would lead to difiiculties in making devices such as shownin FIG. 8 from material such as shown in FIG. 1 or FIG. 9. As a result,a high degree of accuracy may be maintained in the finished product.

(2) By making layers such as 3 and 7 in the trilaminate assembly ofdifferent materials, the former having the higher melting point, a lessbrittle joint of better appearance may be maintained.

(3) By using a thin layer of active metal other than titanium in thetrilaminate assembly, that is to say D36 for example, a very evenconcentration of the same in the alloy can be obtained. This results inan even quality of braze throughout a joint.

(4) By melting the alloy formed by the trilaminate assembly such as (3,5, 7) or (33, 35, 37) only to the viscous state and squeezing it in thisstate to spread in a controlled manner, rather than running away, astronger and superiorly formed joint S is obtained, whether of thevariety such as shown in FIG. 8, or as shown in FIG. 10.

In view of the above, it will be seen that the several objects of theinvention are achieved and other advantageous results attained.

As various changes could be made in the above constructions, methods andproducts without departing from the scope of the invention, it isintended that all matter contained in the above description or shown inthe accompanying drawings shall be interpreted as illustrative and notin a limiting sense.

What is claimed is:

1. The method of forming a brazed joint between objects at least one ofwhich is a metal and another a ceramic, comprising locating between atleast two of the objects components constituted by metals which whenheated to form an alloy will form a bond between at least one of themetal objects and the ceramic object, and simultaneously squeezing saidtwo objects with said components between them while heating only to atemperature which will transform said components while under pressureinto a viscous liquid phase so as controllably to flow on the ceramic,and thereafter cooling to solidify the alloy.

2. The method according to claim 1, wherein squeezing pressure isgradually increased with gradual rise in temperature.

3. The method of brazing a metal part to a ceramic part, comprisingsolid-phase bonding to the metal part a multiple laminate assembly, thelaminations themselves of which are solid-phase bonded and contain amongthem a metal active in an alloy of the same to bond to the ceramic, andsimultaneously squeezing and heating the multiple laminate assemblybetween the ceramic and metal parts only until a temperature is reachedat which the multiple laminate assembly while under pressure forms aviscous liquid-phase alloy so as to controllably flow on the ceramic,and thereafter releasing the pressure and lowering the temperature tosolidify the alloy.

4. The method of brazing a ceramic part to a metal part, comprisingsolid-phase bonding to a sheet of the metal a multiple laminateassembly, the laminations themselves of which are solid-phase bonded andcontain among them a metal active in an alloy of the same to bond to theceramic, mechanically forming the solid-phase bonded sheet and assembly,thereafter simultaneously squeezing and heating the multiple laminateassembly between the ceramic and formed metal parts only to atemperature at which the multiple laminate assembly forms a viscousliquid-phase alloy, and thereafter releasing the pressure and loweringthe temperature.

5. The method of brazing a ceramic part to a metal part, comprisingsolid-phase bonding to a substantially flat supporting sheet of metalwhich is to become part of a laminated metal assembly, laminations whichare solidphase bonded and contain among them a metal active in an alloyof the same to bond to the ceramic, mechanically forming said bondedpart and assembly, and thereafter simultaneously squeezing and heatingthe laminated metal assembly between the ceramic and metal parts onlyuntil a temperature is reached at which the laminated metal assemblyforms a liquid-phase alloy in a viscous state, and thereafter releasingthe pressure and lowering the temperature to solidify the alloy.

6. The method of brazing a ceramic part to a metal part, comprisingbonding a layer of metal which forms the metal part and a bonded groupof metal layers to form an alloy, the latter layers containing amongthem a layer formed of a metal active in the alloy to bond to theceramic, contacting the ceramic part with the outer one of said group oflayers, heating and squeezing said group of layers between the ceramicpart and the metal layer until but only until a temperature is reachedat which a liquid-phase alloy is formed in a viscous state whichcontrollably spreads on the ceramic and thereafter releas ing thepressure and lowering the temperature to solidify the alloy in a bondwith the ceramic part.

7. The method of brazing a ceramic part to a metal part, comprisingassembling and solid-phase bonding first and second metallayers, thefirst one of which is to constitute the metal part, with a group ofmetal layers to form an alloy braze, said group of layers containingamong them a layer formed of a metal active in the alloy to bond to theceramic, contacting the ceramic part with the outer one of said group oflayers, heating and squeezing said group of layers between the ceramicpart and the second metal layer until but only until a temperature isreached at which a liquid-phase alloy is formed in a viscous state so asto controllably spread on the ceramic,

and thereafter releasing the pressure and lowering the temperature tosolidify the alloy in a bond with the ceramic part.

8. The method of brazing a ceramic part to a metal part, comprisingsolid-phase bonding a multiple laminate assembly to a metal supportingpart, the laminations being solid-phase bonded together and containingamong them a metal active in an alloy of the same to bond to the ceramicpart, the laminations having a lower melting point than that of eitherthe metal or ceramic parts, mechanically forming said metal part andlaminate assembly, thereafter contacting the ceramic part with the outerlayer of said laminate assembly, simultaneously heating and squeezingsaid laminate assembly between said metal and ceramic parts until atemperature is reached at which the laminate assembly forms aliquid-phase alloy, and thereafter reducing the pressure and temperatureto solidify the alloy.

9. The method according to claim 8, further comprising solid-phasebonding a buffer metal between the metal part and the laminate assemblyprior to mechanically forming said metal part and laminate assembly,said buffer metal having a coefiicient of thermal expansion intermediatethose of the ceramic and metal parts.

10. The method according to claim 8, wherein squeezing pressure isgradually increased with gradual rise in temperature so as to force thealloy to controllably spread on the ceramic.

11. The method according to claim 8, wherein the parts are heated onlyuntil a temperature is reached wherein the laminate assembly forms aviscous liquidphase alloy so as to controllably spread on the ceramic.

References Cited UNITED STATES PATENTS 1,197,615 9/1916 Eldred 29l95.52,362,893 11/1958 Durst 29504 X 3,031,737 5/1962 Conley 29473.1 X3,068,564 10/1962 Weidt 29-472.3 X 3,209,450 10/1965 Klein et a1.29--501 X CHARLIE T. MOON, Primary Examiner R. J. SHORE, AssistantExaminer US. Cl. X.R. 29-495, 498

